Previous studies from a number of laboratories have demonstrated the presence of cadmium-binding proteins (CdBP) from marine molluscs which exhibit both similarities and differences with mammalian metallothionein (see Environmental Health Perspectives 65:3-224, 1986 for review). Comparative biochemical studies on these proteins are of particular interest both with respect to understanding the regulatory roles of these molecules in toxic-essential metal homeostasis and elucidating possible pathways for metallothionein evolution. The present studies have examined in vivo competition for binding sites on CdBP of the scallop Placopecten magellanicus following exposure to cadmium concentrations of 0, 17.7, 35.4 ppb or 17.7 ppb Cd plus 5.0 ppb Cu (2:1 Cd/Cu molar ratio) in seawater for 8 weeks. Results of these studies indicate that Cd showed dose-related binding to CdBP fractions and that animals receiving the Cd plus Cu dose regimen had approximately two times less Cu than Cd bound to these protein fractions on a molar basis. Competitive radioimmunoassays using polyclonal antibodies to mammalian metallothionein (MT) were used to evaluate possible immunological relationships between MT, the scallop CdBP, and the low molecular weight CdBP from the oyster Crassostrea virginica. Data from these studies indicated that the scallop CdBP was competitive in the RIA relative to mammalian MT suggesting shared antigenic determinants with mammalian metallothionein, while the oyster CdBP which is closer in size to metallothionein was only minimally reactive. Results of these studies suggest that despite an apparent size difference, the 45K CdBP from scallops possess similarities to mammalian MT in regard to both immunological determinants and in vivo regulation of Cd and Cu.